Revealing mistake: When the farmer is going to shoot the sign the bank put up, you can very well see the mark where the hole is going to appear.
Revealing mistake: In the scene where King Arthur expounds on the joy of his kingdom, he has a Band-Aid on the back of his neck. (00:21:00)
Revealing mistake: At the end when the class are having their dance, the band manage to play without any amps, speakers or mikes. We even hear keyboards, despite no-one playing any.
Revealing mistake: When the tank/truck driven by Marvin is crossing the stone bridge at the end, they are met by a small German gun vehicle, which they run off of the bridge. The camera angle changes to below the bridge and you see the German vehicle fall from the side of the bridge into the water below. If you look at the bridge material that shatters you can see pieces of wood and hollow areas where obviously it's not real stone. (02:25:00 - 02:30:00)
Revealing mistake: When Spencer Tracey backs his car into the hotrod at the ice cream stand, you can see the hotrod already has a sizable dent before the car makes contact, probably a result of a previous take.
Revealing mistake: The day after their first date Benjamin drives through the pouring rain to pick up Elaine. If you look closely out the side windows of the car you can see that it's produced by a rain machine, it's actually a beautiful day with the sun shining. (01:07:40)
Revealing mistake: When Old Timer strikes it rich and digs his gold mine, the rock he is digging and mining is obviously not gold, just ordinary rock.
Revealing mistake: When the brown Vanden-plas hits the skip you can see that the cars engine has been removed also the moment of impact is shown twice.